Dry powder inhalers: An overview of the in vitro dissolution methodologies and their correlation with the biopharmaceutical aspects of the drug products

In vitro dissolution testing is routinely used in the development of pharmaceutical products. Whilst the dissolution testing methods are well established and standardized for oral dosage forms, i.e. tablets and capsules, there are no pharmacopoeia methods or regulatory requirements for testing the dissolution of orally inhaled powders. Despite this, a wide variety of dissolution testing methods for orally inhaled powders has been developed and their bio-relevance has been evaluated. This review provides an overview of the in vitro dissolution methodologies for dry inhalation products, with particular emphasis on dry powder inhalers, where the dissolution behavior of the respirable particles can have a role on duration and absorption of the drug. Dissolution mechanisms of respirable particles as well as kinetic models have been presented. A more recent biorelevant dissolution set-ups and media for studying inhalation biopharmaceutics were also reviewed. In addition, factors affecting interplay between dissolution and absorption of deposited particles in the context of biopharmaceutical considerations of inhalation products were examined.